1. Field of the Invention
The invention relates to an optical imaging device, in particular an objective for semiconductor lithography, having at least one system diaphragm, the system diaphragm comprising a multiplicity of mobile plates.
2. Description of the Related Art
The use of various types of diaphragms as system diaphragms in optical imaging devices is widely known. The delimitation of ray bundles by diaphragms is very important for carrying out the optical imaging. Diaphragms can influence or vary the diameter of the ray bundle. The optical imaging quality of the optical imaging device can thereby be substantially influenced and improved.
In semiconductor lithography optical systems, it is known to use iris diaphragms with a flat diaphragm plane. It may, however, be necessary for delimitation of the ray path to be provided for the different aperture widths at different positions along the optical axis. This has already been done by existing diaphragms, as explained below.
For example, DE 199 55 984 A1 discloses a diaphragm in which, beyond a certain aperture width, a further flat plate assembly delimits the ray path in a second position. The diaphragm comprises at least two diaphragms arranged at an axial distance from one another. A different diaphragm is respectively optically active as a function of the aperture diameter of the system diaphragm.
DE 199 55 984 A1 furthermore discloses a diaphragm whose plate assembly can be displaced along the optical axis. The plates are arranged between two rings mobile in rotation relative to each other, at least one of the rings being rotationally mobile. The plate assembly is respectively displaced or moved parallel in the axial direction.
DE 199 55 984 A1 also discloses a diaphragm whose plates are arranged in a conical fashion. The optically effective edge of the diaphragm can therefore be moved, for example over a conical peripheral surface or over a peripheral surface of a spherical cap shape. The plates arranged rotationally symmetrically with respect to the optical axis of the system diaphragm therefore move with a linear or semicircular dependency between the aperture diameter and the axial position in the light path of the imaging device. The disadvantage of such an arrangement is that the cone angle is greatly limited, and the ray path cannot therefore follow the optimal shape of a sphere. Added to this, there are significant friction problems which are caused by bending of the elastic plates.
WO 02/31870 discloses a projection system with aperture diaphragms, the aperture diaphragms being arranged in the vicinity of the pupil plane. At least one of the aperture diaphragms can be modified in its aperture diameter and moved in the axial direction.
It is likewise known from the previously disclosed solutions, for instance from DE 199 55 984 A1, that such diaphragms in lithography objectives contain rolling or sliding bearings. For example, the rotational bearing axes of the plates are mounted in a sliding or rolling fashion and/or the drive element, which moves the plates, is mounted in a sliding or rolling fashion and/or there are sliding or rolling bearings at the force drive point of the plates. Each of such bearings within a lithography objective then constitutes a problem. Sliding or rolling friction can create particles which contaminate the objective and therefore greatly compromise the imaging quality. Lubrication inside the objective can furthermore be highly problematic, although even unlubricated bearings represent a high risk as regards operating reliability.